Stress information about a formation is collected and used to make decisions associated with exploring for hydrocarbons as well as developing and producing hydrocarbons that have been discovered. For example, stress information about the formation is used in activities such as drilling margin estimation, evaluation of wellbore or borehole stability, column height estimation, determining drilling direction, determining lifespans of wells and completions, and waterflooding. If the formation is well understood, exploration, development, and production activities can be planned and executed in a cost effective and efficient manner.
The stress state and magnitudes of stresses of the formation are used in deciding how to approach exploration, development, and production activities. For example, the maximum stress magnitude is used to understand the acoustic properties of reservoirs and mudrock and to determine the porosity and volume of reservoirs. Understanding the acoustic properties of the reservoirs can facilitate increasingly accurate interpretations of acoustic measurements that are used to explore the formation. If the exploration information is accurate, there is less risk in using the information to move forward with development and production activities.
In quantifying the stress state, typically, the overburden pressure is used to determine the magnitude of the vertical stress and a leakoff test is used to determine the magnitude of the minimum stress. The leakoff test is analyzed to determine a leakoff pressure or other pressure that can be interpreted as the pressure when failure of the formation occurs during the leakoff test. The standard assumption is that the formation fails under a tensile mode of failure and the leakoff pressure represents either reopening of existing fractures or the initiation of a new tensile fracture in non-permeable rock. Leakoff pressure is generally considered to be a reasonable estimate of the minimum principle stress of the formation.
However, in certain instances, the assumption that leakoff pressure represents the minimum principle stress of the formation has been found to be inconsistent with other evidence relating to the stress state. For example, in one instance, evidence has suggested that the stress state is a reverse-fault setting or area under compression. Here, for the leakoff pressure to be considered to be reflective of the minimum stress, leakoff pressure should be near overburden pressure. In contrast, well-understood and quality controlled leakoff pressure data was observed to be well below the overburden pressure. Accordingly, leakoff pressure does not always provide a reasonable estimate of minimum principle stress that is consistent with other evidence of the stress state and the standard assumption is not applicable in every case. What is needed is a new method of characterizing formation stresses.